Reagent test strips are widely used in the field of clinical chemistry. A test strip usually has one or more test areas, and each test area is capable of undergoing a color change in response to contact with a liquid specimen. The liquid specimen usually contains one or more constituents or properties of interest. The presence and concentrations of these constituents of interest in the specimen are determinable by an analysis of the color changes undergone by the test strip. Usually, this analysis involves a color comparison between the test area or test pad and a color standard or scale. In this way, reagent test strips assist physicians in diagnosing the existence of diseases and other health problems.
Color comparisons made with the naked eye can lead to imprecise measurement. Today, strip reading instruments employ a variety of area array detection readheads utilizing CCD (charge-coupled device), CID (charge-injection device) or PMOS detection structures for detecting color changes to the test strips. These instruments accurately determine the color change of a test strip but sometimes fail to measure minute color inconsistencies due to the limited spatial resolution for a given sample detection area or field of view. For example, a 739.times.484 pixel array with a fixed field of view equal to 4".times.3" results in a fixed pixel spatial resolution equal to 0.0062".times.0.0054" (based on a pixel size of 9.92 .mu.m.times.8.66 .mu.m). Area array detection readheads having resolutions in this range can fail to detect minute color variations (i.e. Non Hemolyzed Trace detection of Occult Blood reagent, etc.) on the image of a MULTISTIX.RTM. reagent strip of the type sold by Miles Inc., Diagnostics Division, of Elkhart, Ind. 46515. After the urine specimen contacts the test pad of a MULTISTIX.RTM. reagent strip, intact blood cells appear as tiny green blotches on the yellow test area. The area array detection readhead can miss the minute color variation caused by an individual blood cell due to the cell's very small size. Unfortunately, area arrays detection readheads having higher spatial resolutions which can detect minute color variations cost considerably more and are less reliable. Therefore, a need exists for new area array detection system which provides improved spatial resolution without a significant increase in cost or risk.
Most area array detection readheads are designed to work with video camera systems which conform to broadcast television standards, and thus the spatial dimensions (L.times.W) of the area arrays in the detection readheads are proportional to the 4.times.3 aspect ratio of television picture tubes. Area array detection readheads typically use area arrays having the 4.times.3 aspect ratio to lower cost and risk. Typically, the area array detection readhead has a lens that images a fixed sample detection area or field of view (the example above used a 4".times.3" area) onto an area array having a 4.times.3 aspect ratio (i.e. 6.4 mm.times.4.8 mm). The present invention takes advantage of the lower cost and risk associated with current area array detectors and provides improved spatial resolution.